Magnetic particles are known for use in laboratory and industrial procedures in which such particles are transported by applied magnetic fields. Typically, these procedures disperse particles in a liquid and impose a magnetic field on the liquid to magnetize the particles. The magnetized particles are then moved through the liquid by altering the specific orientation of the magnetic field lines with respect to the liquid, and utilizing the natural tendency of the magnetic particles to align and experience magnetic attractions. The ease with which these magnetic particles may be moved within the liquid and subsequently collected using a means of magnetic attraction has led to wide use of this technique to create forced liquid circulations to mix heterogeneous components, facilitate chemical and biological reactions, reduce transfer resistances at or near the free surface of the liquid, and conduct other processes aided by liquid agitation.
Existing methods have largely relied on the tendency of a magnetic particle subjected to an external magnetic field to align itself with the external magnetic field lines in such a way that the magnetic particle stabilizes into the configuration with the lowest energy. The magnetic particle tends to align in opposed polarity to the external magnetic field, and experiences a torque causing rotation of the magnetic particle's magnetic dipole moment by one of two ways: (i) the magnetic dipole moment itself can rotate inside the particle against the internal magnetic anisotropy field, or (ii) the entire magnetic particle can physically rotate, thus keeping the magnetic dipole moment aligned with the internal anisotropy field and the external magnetic field lines. In addition to this rotation, the magnetic particle may also experience movement driven by a magnetic drag force governed by the dipolar fields of any neighboring magnetic particles and the external magnetic field. The existing methods for driving liquid flow using magnetic particles stimulate these effects simultaneously on a multiplicity of magnetic particles, thereby agitating the liquid.
In the existing methods, the external magnetic field source can be one or more permanent magnets, electromagnets or a combination thereof. The magnetic particle may be a permanent magnet or a material which is magnetized by the external magnetic field. The external magnetic field source is established such the external magnetic field lines penetrate through the medium to some degree, and the magnetic particles correspondingly experience rotation and movement as outlined above to establish magnetic dipole moment alignment. The relative spatial orientation between the external magnetic field lines and the medium is then altered by physically relocating the magnetic field source or the medium, or, in the case of an electromagnet, through control of input power. As a result of this altered relative spatial orientation, the magnetic dipole moments of individual particles realign in accordance with the now altered orientation of the external magnetic field lines, resulting in particle movement through the medium. This movement acts to disturb the medium in which the particles are dispersed. Continuous alteration of magnetic field line spatial orientation in this manner produces essentially constant movement of the particles through the medium. (See, U.S. Pat. No. 6,228,268 B1 issued to Siddiqi, issued on May 8, 2001; U.S. Pat. No. 4,936,687 issued to Lilja, et al, issued Jun. 26, 1990; U.S. Pat. No. 6,033,574 issued to Siddiqi, issued on Mar. 7, 2000; U.S. Pat. No. 6,776,174 B2 issued to Nisson, et al, issued on Aug. 17, 2004; U.S. Pat. No. 4,310,253 issued to Sada, et al, issued on Jan. 12, 1982; U.S. Pat. No. 6,616,730 issued to Bienvenu, issued on Sep. 9, 2003). A drawback to these methods is the requirement for essentially continuous alternation of the relative spatial orientation between the external magnetic field lines and the medium, which requires either complex physical apparatus in order to physically relocate the magnetic field source or the medium, or, in the case of an electromagnet, intricate timing mechanisms to vary input power in a predetermined manner. Additionally, since the liquid agitation rate in these systems depends directly on the rate at which the relative spatial orientation can be altered, any complex physical apparatus relied on to physically relocate the AC magnetic field source or the medium fades severe limitation as the rate is increased.
Other methods for liquid agitation using magnetic particles utilize self-assembled solid-state structures to aid the agitation. These methods establish a magnetic particle density and an external magnetic field strength such that, as magnetic particles in the liquid approach each other and experience dipole-to-dipole attraction, the magnetic particles self-assemble into solid-state-structures. These solid-state structures possess a characteristic magnetic moment, and as the external magnetic field orientation is altered, the structures realign with the altered magnetic field lines and thereby move through the medium. Typically, as the structures move through the medium, they periodically break apart into component magnetized particles, which then experience additional dipole-to-dipole attractions sufficient to result in the self-assembly of new structures with characteristic magnetic moments. In these methods, the self-assembly of magnetic particles is deliberately provoked in order to increase the agitation of the medium as self-assembled structures rather than individual magnetic particles move through the medium (See U.S. Pat. No. 5,222,808, issued to Sugarman, et al, issued on Apr. 10, 1992; U.S. Patent Application No. 2007/0207272 A1, submitted by Pun, et al, published Sep. 6, 2007; U.S. Patent Application 2007/0036026 A1, submitted by Laibinis, et al, published Feb. 15, 2007). These methods offer advantage in some situations, however they still rely on magnetic dipole alignment with external magnetic field lines, and still require essentially continuous alternation of the relative spatial orientation between the external magnetic field lines and the medium. As a result, they retain the drawback of requiring either complex physical apparatus in order to physically relocate the magnetic field source or the medium, or, in the case of an electromagnet, intricate timing mechanisms to vary input power in a predetermined manner.
Snezhko, et al., has reported the formation of self-assembled structures which oscillate around stationary positions on the liquid surface and produce highly stable, localized, stationary vortex flows, with essentially dead flow areas existing outside the stable vortices. These self-assembled structures are produced by suspending magnetic particles on a free surface, and subjecting these particles to a traverse AC magnetic field. See “Surface Wave Assisted Self-assembly of Multidomain Magnetic Structures,” Physical Review Letters, vol 96, Issue 7, (February 2006), and, “Dynamic self-assembly of magnetic particles on the fluid interface: Surface-wave-mediated effective magnetic exchange,” Physical Review E, vol 73, 041306 (April 2006), “which are hereby incorporated by reference in their entirety. However, Snezhko, et al., limits his discussion to low frequency regimes where self-assembled structures oscillate around essentially stationary points on the free surface, producing stable liquid vortices with dead flow areas outside the vortices where mixing is severely compromised.
What is presented here is a novel method of driving liquid flow at or near the free surface of a liquid by utilizing non-stationary, self-assembled structures which independently move across the free surface. During the course of this movement, the oscillations of the self-assembled structures produce a series of unstable temporary vortices at or near the free surface. In this manner, vortices are transitorily created across essentially the entirety of the free surface and dead areas are essentially eliminated. This method utilizes a traverse AC magnetic field with a fixed orientation between the AC magnetic field source and the liquid, avoiding the need for complex physical apparatus or intricate timing mechanisms, and allows precise, repeatable control of liquid flow velocities through selection of magnetic field frequencies. The method has use for various purposes, including but not limited to mixing heterogeneous components, facilitating chemical and biological reactions, reducing transfer resistances at or near the free surface of the liquid, or other process aided by liquid agitation